PROJECT SUMMARY (ABSTRACT) Risk for ischemic stroke, a leading cause of death and disability in the United States, is primarily assessed to- day by degree of carotid artery stenosis. However, insufficiencies in stenosis degree as a biomarker for stroke risk commonly lead to unnecessary surgical revascularization by carotid endarterectomy (CEA). It is estimated that, among patients with clinical indication for CEA, only 1 out of 14 with 50-69% stenosis experiencing neuro- logical symptoms, and 1 out of 22 asymptomatic patients with 70-99% stenosis, would have had a stroke if CEA was not performed. There exists an urgent yet unmet need to improve differentiation of patients at low risk of embolic stroke from those in need of CEA to prevent it. This need could be met by a technology capable of noninvasively interrogating biomarkers that have been established to confer increased risk of stroke and transient ischemic attack (TIA), namely: thin or ruptured fibrous cap (TRFC), lipid-rich necrotic core (LRNC), and intraplaque hemorrhage (IPH). These carotid plaque features have been delineated by MRI and associ- ated with current and future risk for stroke or transient ischemic attack (TIA) in several clinical studies; how- ever, MRI is prohibitively expensive for screening and routine monitoring. The lack of a low-cost, noninvasive imaging method that reliably delineates carotid plaque structure and composition and is suitable for wide- spread diagnostic application represents a major gap in improving stroke risk stratification. To fill this gap, our group has spent the first five-year funding cycle of this grant award developing Acoustic Radiation Force Im- pulse (ARFI) ultrasound for delineating the structure and composition of human carotid atherosclerotic plaque, in vivo, with spatially-matched histological validation. Our research has demonstrated that ARFI delineates LRNC/IPH, collagen/calcium deposits, and TRFC in human carotid plaque, in vivo, with TRFC thickness meas- urement as low as 0.49 mm - the mean thickness associated with carotid plaque rupture. The success of our investigations thus far motivates further advancement of ARFI technology to enhance carotid plaque imaging. Specifically, we will exploit ARFI Variance of Acceleration (VoA) imaging22,23, higher center frequencies24, and harmonic imaging25 to newly enable separate discrimination of TRFC, LRNC, and IPH and accurate feature size measurement. We will determine the association between advanced ARFI?s plaque characterization and recent history of ipsilateral stroke or TIA. We hypothesize that advanced transcutaneous ARFI ultrasound de- lineates TRFC, LRNC, and IPH in human carotid atherosclerotic plaques in vivo, and these ARFI-derived plaque features are associated with recent ipsilateral stroke or TIA. The research team is exceptionally well prepared to pursue this work; the PI and all Co-Is previously collaborated on the successful completion of the first five years of this research program, and skilled support for the design and execution of clinical trials is available through the North Carolina Translational and Clinical Sciences (NCTraCS) Institute at UNC Chapel Hill.